added forgotten NeutronKiller process, cleanup comments
Merge pull request #4493 from civanch/bgp_mctruth_process_num

SimG4Core/Physics -- Added forgotten NeutronKiller process, cleanup comments
Merge pull request #4393 from cms-sw/CMSSW_7_1_X

Forward port CMSSW_7_1_X into CMSSW_7_2_X